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Cleaning methods

1. Immersion or dip cleaning

Immersion cleaning is an old cleaning method. The cleaners used in such processes easily dissolve oils and greases and reliably disperse dirt particles and contaminants. Multi-stage immersion or dip cleaning is recommended for bulk goods but also for precision and fragile parts which must not be damaged.

High-value components such as engine and gearbox housings, fuel injectors, ABS parts, ophthalmic lenses, etc.) are cleaned in special racks to avoid damage.

In many cases, just one immersion is not enough so that, depending on the demands, multi-stage cleaning operations have to be used which include pressure flooding, ultrasound, etc.

In earlier times, simple immersion or dip degreasing equipment consisted of a heatable, sheet steel container in which the components were boiled. The agitation and circulation generated by the boiling solution offered an additional mechanical cleaning effect. This method is no longer used in advanced industrial countries. In today’s equipment, the disadvantages of degreasing by boiling such as heat losses, plumes of vapour, spluttering or overboiling are avoided.

The dip cleaning effect can be mechanically assisted by circulating the cleaning solution or by pressure flooding. In the case of pressure flooding, the solution is pumped at pressures of between 8 and 25 bar through facing and offset nozzles into the cleaning tank and thus over the components.

Alternatively, the cleaning bath can be repeatedly filled and emptied via a flooding pump and filtration devices. A special surface skimming system ensures that any oil or grease floating to the top is removed via an overflow and stops the cleaned components being re-contaminated with demulsifyed oil when they are lifted out of the immersion tank.

Compared to the rather old fashioned boiling processes, immersion or dip cleaning with good circulation offers the additional benefit that optimum cleaning can be achieved at relatively low temperatures of between 50 and 75 °C and in relatively short times. Today’s degreasing equipment no longer releases water vapour into working areas. This method is predestinated for fine and ultra fine cleaning. The use of compressed air in immersion cleaning operations is, if anything, counterproductive because this severely restricts the use of effective cleaners.

The use of ultrasound is almost indispensable if today’s demands on residual contamination and particle size are to be met. In many industrial sectors, for example, the manufacture of precision parts (such as injectors, ABS valves, etc.), ophthalmic lenses and equipment as well as the production of high-precision stainless steel pipes, fittings and components, surface cleanliness standards are extremely high.

The complete removal of used materials and abrasion from components is expensive because such contaminants adhere strongly to metal and glass surfaces. In most cases, treatment with a water-based cleaner solution alone is not sufficient if no additional mechanical support is used. In such cases, ultrasound achieves better results than flooding or spraying. Ultrasonic cleaning utilizes high-frequency acoustic vibration generated by special transducers.

Ultrasound is generated and enters the cleaning fluid as waves and causes the rapid formation and collapse of tiny minute bubbles, a phenomenon known as cavitation. These cavitation bubbles then implode against the surface of any immersed object with an enormous release of energy, sometimes with a pressure of more than 1,000 bar. The microscopic roughness of the component’s surface and the contamination particles act as cavitation nuclei.

This method is particularly suitable for inaccessible areas, drilled holes, embossed surfaces and for special applications where no optimum results can be achieved with conventional cleaning solution processes. In clear solutions, it is possible to see how the imploding bubbles lift contaminants and dirt from surfaces in just a few seconds.

The frequencies used are mostly between 20 and 45 kHz but can be in excess of 100 kHz. The normal treatment duration is I – 3 minutes.

2. Spray cleaning

This cleaning method consists of one or more matched step(s). The powerful mechanical cleaning effect of the high-pressure cleaner jet offers the advantage that this type of treatment is often shorter than immersion or dip degreasing and can be performed at lower fluid temperatures of between 40 and 70 °C.

Potentially, it can use less cleaner volume and a lower detergent concentration of between 10 and 30 grams per liter water compared to immersion cleaning. This can result is:

Spray cleaning lines can range from very simple to highly complex designs. Large quantities of simple components can be cost-efficiently cleaned in spray cleaning lines. And here again, today’s designs no longer release water vapour or mist into the surrounding areas of such lines.

The determining factor for the cleaning effect’s intensity is the shape and arrangement of the nozzles which have to ensure that components are sprayed from all sides and no areas are left untouched by the jets. In simple lines, the fluid pressure is between 0.5 and 4 bar. Considerably higher pressures (> 100 bar) are used for certain cleaning operations, such as for deburring, and for high-pressure cleaning.

There are single-chamber lines, in which the parts to be cleaned are sprayed from all sides on mobile racks as well as continuous, multi-chamber lines which use a type of conveyor system. The principal benefit of the latter in comparison to immersion or dip cleaning is that throughput is higher and cost efficiencies can be achieved. The number and size of the chambers in continuous lines have to be matched to the specific requirements. Nozzle design also depends on the individual function of each chamber.

The use of rapidly demulsifying cleaners is an important prerequisite for the reliable and effective operation of spray cleaning lines. The same equipment as is used to maintain the cleaners in immersion or dip cleaning lines (such as filters, oil separators, skimmers. etc.) should also be used to increase the service life of the spray cleaning solutions.

Spray cleaning products have to be formulated differently to immersion or dip cleaners. The most important difference is foaming behaviour. Spray cleaning products are normally low-foaming or non-foaming above the so-called clouding point! This clouding point is dependent on the tensides used and is often around 40 °C. It should be noted that the information contained in the product’s Product Information sheet should be observed!

That means that the recommended operating temperature of the cleaning fluid must first be reached before new mixtures are used in cleaning lines or after longer periods of downtime, to avoid undesirable foaming after start-up.

Cleaning table

In many smaller workshops or manufacturing plants, machined components are still cleaned manually with a brush. In the meantime however, it is possible to simplify these operations. Instead of solvent baths, brushes and cleaning cloths, cleaning tables with integrated brushes and cleaner hoses are now used. In many cases, the solvent can be replaced with water-based, high performance cleaners which are usually better suited to the cleaning of service and repair parts.

Manual spraying

Often, cleaning is performed by spraying, such as when vehicle engines or machinery which cannot be transported, are cleaned. In this, the cleaning solution is sprayed on the parts in question and allowed to soak in. Very dirty and heavily contaminated surfaces are often treated with undiluted cleaning concentrate. Finally, the dirt and contaminants are hosed-off with a powerful water jet. The treated surfaces are then left to air dry. However, the drying process can be accelerated if compressed air is blown on to the surfaces.

Steam cleaners or high pressure washers

Manual cleaning can also be performed with steam cleaners or high pressure washers which normally do not use solvent-based but water-based cleaning concentrates. The operating principle involves cleaning concentrate being mixed with the steam or water. This mixture is then sprayed on the surfaces to be cleaned at high pressures, whereby the temperature and nozzle pressure to be adjusted to the requirenments of the application in question.

4. Solvent-based cleaners

A solvent is a substance which can dissolve or dilute gases, fluids or solids without a chemical reaction occurring between the dissolved substance and the solvent. According to the TRGS (Technical Rules for Hazardous Substances) definition, solvents are volatile organic solvents with a boiling point of up to 200 °C.

Solvent-based cleaners are usually only used when no contact between the metal and water should occur. Hydrocarbons are noteworthy for their excellent solubility of all types of organic contaminants and their low surface tension which allows them to penetrate crevices, small holes and threads.

As opposed to water, encapsulated solvent lines do not cause corrosion because hydrocarbons are practically inert. Solvents can be easily recycled by distillation and no waste water is produced. If no quantitive recyling is performed, they are, as volatile organic compounds (VOC), harmful to the environment and health and should therefore be used in sealed vacuum cleaning lines. This notwithstanding, working with solvents in open containers poses the risk of fire because of their low flashpoint.

Dearomatized white spirits have steadily replaced aromatic white spirits and are now an important group of organic solvents. Even with significantly reduced environmental, health and safety (EHS) risks, their solvent characteristics are still nearly as good. Typically, their aromatic content is considerably under 1 %.

They are mostly used for the cleaning and degreasing of very greasy or oily service and repair parts such as engines, gearboxes and larger machinery (locomotives). Such products are mainly hydrocarbons and, depending on the intended application, contain additives to improve corrosion protection or to easy washing-off.

Manually applied, aromatic-free, solvent-based cleaners are also highly effective for a large range of applications. Depending on their formulation, they can be used as pure concentrates or as solutions in petroleum, or even to some extent, in water. After a soaking-in period, such cleaners are carefully washed-off with a powerful jet of water. When solvent-based cleaners with separating properties are used, the emulsion created when washed off with water rapidly separates into an oil phase and a water phase so that the oil phase can be reliably removed by an oil separator.

Alkoxypropanols are synthetic solvents, commonly known as modified alcolhols, which contain alcohol and ether groups. As a result of their good balance between polar and non-polar characteristics, these solvents have an ideal profile for a broad range of cleaning tasks because they have the ability to dissolve both polar and non-polar substances.

As these are pure substances, or at least defined mixtures of substances with very narrow boiling points, alkoxypropanols can easily be reconditioned by distillation in metal cleaning lines insofar as the boiling points of the dislodged contamination and dirt are not in the alkoxypropanol range.